Modern motor vehicles frequently include a hill hold control (HHC) which automatically keeps the vehicle on a gradient without the driver having to apply the brake for this purpose. Hill hold systems generally function as follows: During a braking operation in which the driver decelerates the vehicle to a stop, the brake pressure prevailing on the wheel brakes is blocked with the aid of shift valves—normally the so-called switching valves. For this purpose, the valves are activated by a control unit, in which the HHC function is stored, using an appropriate current. The valve current set at the valve determines the level of the brake pressure which the relevant valve is capable of maintaining. If the differential pressure dropping at the valve is greater than the pressure threshold value, medium flows into the valve. It is thus possible to maintain a predefined pressure in the wheel brakes, even if the driver's foot is removed from the brake pedal. To release the brakes, the driver must normally depress the accelerator pedal, or the HHC function is automatically deactivated when the brake pedal has no longer been depressed for, for example, approximately 2 seconds.
In the case of the hill hold systems known from the related art, the valves are normally not closed until the driver has already partially released the brake pedal and the brake pressure prevailing at the master brake cylinder has dropped to a predefined target pressure. To be able to close the valves at the above-named point in time, the brake pressure prevailing at the master brake cylinder is monitored with the aid of a pressure sensor, the so-called pre-pressure sensor. The target pressure is normally a pressure which is at least high enough to hold the vehicle securely at a standstill.
Known hill hold systems therefore require at least two sensors, namely, a sensor for determining the road gradient and a pressure sensor for monitoring the brake pressure in a brake circuit. Accordingly, common hill hold systems are relatively complex and are in particular too expensive as optional equipment in vehicles in the lower price category.